1. Field of the Invention
This invention relates to optical systems, and more specifically, to an optical system incorporating a polarizing element within a Fabry-Perot resonator to improve performance of the system.
2. Description of the Related Art
Resonator-enhanced optical inspection systems, storage devices and other optical systems, such as those described by U.S. Pat. Nos. 6,653,649, 6,700,840, 6,714,295, 6,717,707, 6,778,307 issued to Applicant Clark and others, the specifications of which are incorporated herein by reference, provide improved resolution, surface detection and other performance improvements in traditional optical systems and provide new types of optical systems that were not available prior to the inventions disclosed therein. Specifically, the incorporation of a Fabry-Perot resonator in the above-mentioned optical systems has increased the sensitivity of a particular measurement parameter via the resonance effects, and further made it possible to detect certain optical conditions using an intensity detector, whereas an external interferometer was previously required for the measurement.
When incorporating a Fabry-Perot resonator into such systems, often the only available external measurement location is the point of introduction of the illumination beam. In particular, where the Fabry-Perot resonator is formed by a partially reflective surface interacting with the surface to be measured, which might be reflective and not transmissive, then transmission through the resonator is not measurable at all. The only available measurement point is at the partially reflective surface and therefore only the reflection from the Fabry-Perot resonator can be measured.
Intensity measurements of the reflection are much more difficult and prone to error than transmission measurements. Because the field is “bright” between the resonances (i.e., the cavity re-radiates all wavelengths other than the resonant wavelengths), the background level is a function of the mirror efficiencies (reflectivity and absorption) and the power of the input beam. For relative intensity measurements, use of the reflected beam requires measuring a typically non-zero resonance value lower than the background and comparing it to the above-described bright value, which is referred to as “bright field” detection. Transmission measurements are much simpler in that the values between the resonances are near zero and the resonance peak “bright” values are more easily compared to other values near resonance, which is referred to as “dark field” detection.
Further, when the detection and illumination beams are co-located or overlapped at a partially-reflective surface of a modified Fabry-Perot resonator and the illumination and detection areas are imaged onto each other, such as in the lens-incorporating resonator of the above-incorporated parent U.S. patent application, cross-talk between reflections across the image will affect performance unless measures are taken to ensure that the reflections do not interfere.
It would therefore be desirable to improve the performance of the Fabry-Perot resonator-enhanced optical systems disclosed in the above-referenced U.S. Patents, as well as other optical systems, in order to further improve their resolution and performance. It would further be desirable to provide a mechanism for observing a resonance value in a dark field (i.e., bright resonance on dark background) at the illumination input to the resonator. It would also be desirable to provide a mechanism whereby illumination and detection beams can co-located and still yield ideal resonator performance without perfect optics and illumination.